<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        /*
        
        */
        *{
            margin: 0;
            padding: 0;
        }
        html,body{
            height: 100vh;
        }
        .content{
            height: 100vh;
            background-color: red;
            position: relative;
        }
        .box1{
            height: 100px;
            background-color: green;
        }
        .box2{
            position: absolute;/*离开文档流*/
            top:100px;
            bottom: 0px;
            /*width: 100vw;*/
            right: 0;
            left: 0;
            background-color: yellow;
        }
    </style>
</head>
<body>
    <div class="content">
        <div class="box1"></div>
        <div class="box2"></div>
    </div>
</body>
</html>